In seismic exploration, geophysical data are obtained by applying acoustic energy to the earth from an acoustic source and detecting seismic energy reflected from interfaces between different layers in subsurface formations. The seismic wave-field is reflected when there is a difference in acoustic impedance between the layers on either side of the interface.
Marine seismic prospection is generally made with seismic streamers (also referred to as “linear acoustic antennas”) towed through water, behind a recorder vessel at a water depth normally between about six to about nine meters, but can be towed shallower or deeper. The streamers support pressure sensors such as hydrophones to detect seismic signals corresponding to pressure waves. Seismic sources may be also towed behind the recorder vessel. Seismic sources may be for example air gun arrays or water gun arrays or other sources known to those skilled in the seismic art.
Alternatively, the seismic streamers are maintained at a substantially stationary position in a body of water, either floating at a selected depth or lying on the bottom of the body of water, in which case the source may be towed behind a vessel to generate acoustic energy at varying locations or the source may be maintained in a stationary position.
Multi-component streamers usually use at least two nearly collocated sensors (or group of sensors): one pressure sensor (e.g. hydrophone), or a group of pressure sensors, and at least one particle motion sensor (e.g. geophone or accelerometer), or a group of particle motion sensors. The at least one particle motion sensor (or the group of particle motion sensors) is nearly collocated to the pressure sensor (or the group of pressure sensors).
While the hydrophone is an omnidirectional sensor and so, does not need to be oriented, the particle motion sensors measure the amplitude of the wave (speed or acceleration of the particle) on a given direction. To do so, the orientation of the particle motion sensors must be known.
Knowing that it is nearly impossible to predict the rotation of the streamer in water, there are usually two possible solutions to know said given direction, as discussed below.
A first known solution consists in mechanically insuring that the particle motion sensor is in a known orientation, using for example gravity. One way to perform this is to ballast the particle motion sensor and a gimbal mounts the particle motion sensor in a housing filled with lubricant damping fluid.
This first known solution has the main disadvantage of affecting the particle motion sensor response, as the motion of the sensor induced by streamer rotation is biased by the gimbal arrangement (inertia, friction, etc.). Moreover, such gimbal mounting is usually complex, by involving additional mechanical parts, and takes too much space in the streamer.
A second known solution is to create a 2 or 3-dimension particle motion sensor base and to use a nearly collocated tilt sensor, with a known orientation compared to this base. The tilt measurement is then used to recover the vertical, the cross-line, or the inline component of the particle motion wave. This can for example be implemented through a MEMS (“micro-electro-mechanical system”) device, that can measure at the same time the tilt and the acceleration.
An example of this second known solution is now further described with FIGS. 1 and 2.
FIG. 1 presents an example of a portion of a multi-sensor streamer 2 comprising a group of several sensors which are nearly collocated (i.e. in close proximity): a pressure sensor (e.g. hydrophone) H, a particle motion sensor (e.g. geophone or accelerometer) PMS and a tilt sensor (e.g. a MEMS device) TS.
(X0,Y0,Z0) is the reference right-handed coordinate system. X0 (reference horizontal axis) is considered as collinear with the longitudinal axis 1 of the streamer 2 and points towards the tail end of the streamer. Z0 (reference vertical axis) is collinear with the gravity vector g (true vertical position) and points towards the earth (Z0=g).
The particle motion sensor PMS is for example a 2-axis accelerometer that measures seismic signal.
If the sensor cannot measure the static acceleration (i.e. gravity) (e.g. a piezo capacitive sensor), therefore it cannot measure directly the angular position. The particle motion sensor of FIG. 1 has two orthogonal sensing axes Y and Z, in a plane orthogonal to the longitudinal axis 1 of the streamer 2. The tilt angle β of the particle motion sensor PMS (also referred to as the angular position of the particle motion sensor) is formed by the Z sensing axis and the gravity vector (Z0=g) (see FIG. 2).
As the tilt angle β of the particle motion sensor PMS cannot be measured directly, the role of the tilt sensor TS is to measure the tilt angle α of the tilt sensor (also referred to as the angular position of the tilt sensor). The tilt sensor of FIG. 1 has two orthogonal sensing axes Y′ and Z′, in a plane orthogonal to the longitudinal axis 1 of the streamer 2. The tilt angle α of the tilt sensor is formed by the Z′ sensing axis and the gravity vector (Z0=g) (see FIG. 1).
The tilt angle of the particle motion sensor versus time can be written as:
β(t)=β′(t)+α(t), with β′(t) the angular offset between the particle motion sensor PMS and the tilt sensor TS.
The locations of the tilt sensor TS and the particle motion sensor PMS are very close, that's why we can consider that they have an identical rotational motion. Therefore β′ can be considered as constant: β(t)=β′+α(t). In general, β′ is calibrated during the manufacturing and the system is able to calculate β(t) when α(t) is measured.
Therefore the system of FIGS. 1 and 2 operates as follows: it first records the acceleration with the sensing axes Y and Z (the corresponding acceleration data are noted respectively AccY(t) and AccZ(t)) and measures α(t). Knowing β′, the system is able to calculate β(t).
The final acceleration data (noted AccY0(t) and AccZ0(t)) have to be expressed in the coordinate system (X0,Y0,Z0). To this end, the system performs a rotation (around X0) of β(t) of the acceleration data AccY(t) and AccZ(t), according to the following formula:
      (                                                      AccY              0                        ⁡                          (              t              )                                                                                      AccZ              0                        ⁡                          (              t              )                                            )    =            (                                                  cos              ⁢                                                          ⁢                              β                ⁡                                  (                  t                  )                                                                                        sin              ⁢                                                          ⁢                              β                ⁡                                  (                  t                  )                                                                                                                        -                sin                            ⁢                                                          ⁢                              β                ⁡                                  (                  t                  )                                                                                        cos              ⁢                                                          ⁢                              β                ⁡                                  (                  t                  )                                                                        )        ×          (                                                  AccY              ⁡                              (                t                )                                                                                        AccZ              ⁡                              (                t                )                                                        )      
The system records simultaneously the pressure P(t) through the pressure sensor (e.g. hydrophone) H at the same location. The pressure sensor H is nearly collocated (i.e. is in close proximity) with the particle motion sensor PMS.
The second known solution solves the main disadvantage of the first known solution (see above), but it has the drawback of requiring a tilt sensor TS (which is an additional sensor), at the particle motion sensor's location, and its associated power.
The angular offset β′ between the particle motion sensor PMS and the tilt sensor TS needs to be known precisely. For now, this requires a calibration procedure during manufacturing. This kind of procedure is often costly.
The angular offset β′ has to be constant. For now, there is no easy mean to check the value (accuracy) of this angular offset β′ while the streamers are at sea.
Moreover there is no mean to check the quality of the tilt angle α of the tilt sensor while the streamers are at sea.
Requiring an additional sensor also means more wires in the streamer and so, some impact on the overall weight and size of the streamer.
Document GRATACOS B: “Reorientation and calibration of nongimbaled multicomponents sensors”, ANNUAL INTERNATIONAL SEG MEETING, EXPANDED ABSTRACTS, TULSA, Okla., US, 26 Oct. 2803 (2883-18-26), pages 838-841, referred hereinafter as GRATACOS, deals with reorientations and calibration method for non gimballed multicomponents sensors. However said sensor are supported by a cable that is laid out on the sea floor. Such a cable is usually referred as an Ocean Bottom Cable, and is not intended to be towed nor to move once laid on the sea bed, conversely to a streamer.
The method for determining orientation of the sensors according to GRATACOS uses a seismic signal resulting from a shot of a seismic source. However a seismic signal resulting from a shot of a seismic source is a punctual event corresponding in the example of GRATACOS to a time-window of 500-700 msec. Such time-window is not adapted to determine continuously the tilt angle of a sensor supported by a streamer. Furthermore, using a seismic signal to determine the orientation of a sensor according to GRATACOS method implies to solve ghost issues that lead to a complex method.
There is thus a need in the art for reliably and simply determining orientation of a sensor supported by a towed streamer.